(1) Field of the Invention
The present invention is directed to non-destructive testing and more particularly to a method for detecting microscopic structures within a specimen.
(2) Description of the Prior Art
Optical microscopes are commonly used to discern small features embedded in specimens. Electron microscopes are also used to discern small features which can be imaged. Electron microscopes exhibit a significantly lower diffraction limit than optical microscopes because the wavelengths of electron beams are much smaller than wavelengths of light. However, electron microscopes are much more expensive than optical microscopes and further require that the specimens be tested in a vacuum.
Photothermal frequency scan imaging is used for materials analysis in semiconductor manufacture. In photothermal frequency scan imaging a sample is subjected to a pulsed laser. This can be used to detect surface conditions of the sample, but it has only been found to be effective for a limited depth in the sample. These methods have had problems with internal reflections in smaller samples. These methods have also been used as an indication of homogeneity within a sample and also for determining thermal properties within the sample. These methods have not been used to image individual discontinuities within a sample.